Piston assembly



United States Patent I 3,402,645 PISTON ASSEMBLY Loren C. Nelson,Middleburgh Heights, Ohio, assignor to gnipin Carbide Corporation, acorporation of New or a Filed Oct. 3, 1966, Ser. No. 583,874

7 Claims. (Cl. 92258) This invention relates to a novel piston assemblyfor use in a high temperature die casting apparatus. More particularly,it concerns a refractory piston assembly which is especially suited foruse in a molten aluminum containing environment.

The die casting art is replete-with apparatus for die casting lowmelting point metals such as lead, tin, and zinc. However, it disclosesa limited number of apparatus which are suitable for die casting highmelting point metals such as aluminum and magnesium.

In copending application Ser. No. 320,693, assigned to the sameassignee, there is described an apparatus for efiiciently andeconomically die casting such high melting point metals. Briefly, thedie casting apparatus described therein comprises a pot containingmolten metal to be cast, a substantially U-shaped gooseneck having aninternal passageway therethrough suspended in the pot of molten metal, adie cavity communicating with one end of the goosenecks internalpassageway, a shot cylinder mounted upon the opposite end of thegooseneck and a piston positioned in the shot cylinder which whenadvanced into the shot cylinder forces the molten metal therein to movefrom the shot cylinder through the gooseneck into the die cavity.

In the before-described type of die casting apparatus, it is essentialthat the piston be fabricated from a material which is highly resistantto the erosive and corrosive action of the metal being cast.

It is well known that titanium diboride is essentially chemically immuneto the action of molten metals such as aluminum and the like. However,it is equally well known that titanium diboride possesses certaininherent physical characteristics which often limit or prohibit its useas a material of construction.

For example, it is well known that although titanium diboride ischaracterized by its high compressive strength its use is oftenprohibited or limited due to its inherent low tensile or flexurestrength.

Accordingly, it is the principal object of the invention to provide animproved piston assembly which is adapted for use in a high temperaturedie casting apparatus.

Another object of the invention is to provide a refractory pistonassembly which is ideally suited for use in an apparatus for die castingmolten aluminum.

A still further object of the invention is to provide a means ofemploying titanium diboride as a material of construction in thefabrication of a piston assembly adapted for use in an apparatus for diecasting high melting point metals such as aluminum and the like.

With the foregoing and other objects in view, the invention will now bedescribed in greater particularity and with reference to the soledrawing which is a cross sectional view of the piston assembly of theinvention. Referring to the drawing, there is shown the preferredembodiment of the piston assembly of the invention. This assemblycomprises a piston sleeve 10 having an annular opening therethrough, apiston tie rod 14 having a front and a rear end axially positioned inthe annular opening in the piston sleeve, a front end cap 12 attached tothe front end of the piston tie rod which restrains the forward movementof the piston sleeve, a threaded rear locking nut 16 attached to therear end of the piston tie rod which restrains the rearward movement ofthe piston sleeve and a shank member 18 which is connected to the rearend of the piston tie rod and adapted at 30 to engage a means ofreciprocating the piston assembly when it is mounted in a die castingapparatus. Resilient sealing members 20, 22, and 24 are positionedbetween the front end cap and the piston sleeve, the rear locking nutand the piston sleeve, and the rear locking nut and the shank member,respectively, to prevent the ingress of molten metal into the annularopening in the piston sleeve during the die casting operation. Thepiston tie rod and the front end cap are provided with registered or inline holes which are designed to receive a front locking pin 26 whichprevents the front end cap from unscrewing from the piston tie rod.Likewise, the shank member and the rear end of the piston tie rod arealso provided with registered or in line holes which are adapted toreceive and hold a rear locking pin 28 which prevents the shank memberfrom unscrewing from the rear end of the piston tie rod.

The piston sleeve is composed of titanium diboride and the piston tierod of a material which has a lower coefficient of thermal expansionthan that of titanium diboride. This material preferably is molybdenum.However, other materials such as tungsten and the like may also beemployed. By so-fabricating the piston tie rod, in actual operation, itis possible to keep the piston sleeve under a continuous compressiveload as the piston sleeve expands more than the piston tie rod and isaccordingly squeezed between the front end cap and the rear locking nut.This type of piston assembly enables one to take advantage of titaniumdiborides excellent compressive strength.

The front end cap, the rear locking nut and the shank member can becomposed of any material which does not react with and is resistant tothe metal being cast. In actual operation, it has been found desirableto fabricate both the front end cap and the rear locking nut ofmolybdenum and the shank member from machine steel. The front and rearlocking pins were also made of machine steel. The sealing members werering shaped and composed of expanded graphite produced as described incopending US. patent application Ser. No. 273,245, entitled ChemicalProducts and Processes, which is also assigned to the same assignee asthe present invention. However, other sealing or gasketing materialssuch as asbestos, glass fiber, carbon and graphite textiles and the likecan be used with equal success. In fact, it is not necessary to use asealing member at the various locations shown in the drawing if thecorresponding mating parts are provided with lapped surfaces.

The average longitudinal coefiicient of thermal expansion of thetitanium diboride piston sleeve in the room temperature to 650 C. rangewas calculated to be 7.75 l0 while that of the molybdenum piston tie rodwas computed to be 6.4X1O- It is this difference in thermal expansioncoefiicients which cause the titanium diboride piston sleeve to besqueezed or held in compression between the front and rear end caps.

It should be noted that it is possible to omit the rear locking nutshown in the drawing and still have a piston assembly which accomplishesthe objects of the invention. This is achieved by extending the lengthof the shank member and bring it into contact with the rear portion ofthe piston sleeve.

The foregoing disclosure is posed for illustrative purposes only andthere is intended here no unwarranted restrictions or limitations on thepiston assembly of this invention, it being considered that in light ofthis disclosure numerous alternative embodiments will suggest themselvesto persons familiar with the art, The intended limitations are thosestated in the appended claims.

What is claimed is:

1. A piston assembly for use in an apparatus for die casting a highmelting point metal comprising:

(a) a piston sleeve composed of titanium diboride, said piston sleevehaving an annular opening therethrough;

(b) a piston tie rod composed of a metal having a lower coefiicient ofthermal expansion than the titanium diboride piston sleeve extendingthrough said annular opening in said piston sleeve, said piston tie rodhaving a front and a rear end;

(c) a front end cap attached to the front end of said piston tie rodwhich is adapted to restrain the forward movement of said piston sleeve;and

(d) a means for restraining the rearward movement of said piston sleeveso that when said piston assembly is exposed to increasing temperaturessaid piston sleeve axially expands more than said piston tie rod therebycausing said piston sleeve to be maintained in compression between saidfront end cap and said means for restraining the rearward movement ofsaid piston sleeve.

2. The piston assembly of claim 1 wherein said piston tie rod iscomposed of molybdenum.

3. The piston assembly of claim 1 wherein a resilient, refractory gasketis positioned both between said front end cap and said piston sleeve andsaid means for restraining the rearward movement of said piston sleeve.

4. The piston assembly of claim 3 wherein said resilient, refractorygasket is composed of expanded graphite.

5. The piston assembly of claim 1 wherein said means for restraining therearward movement of said piston sleeve comprises:

(a) a rear locking nut attached in close proximity to the rear end ofsaid piston tie rod adapted to restrain the rearward movement of saidpiston sleeve; and

(b) a shank member which engages the end of said piston tie rod and isadapted to communicate with a means for reciprocating said pistonassembly.

6. The piston assembly of claim 5 wherein a resilient,

refractory gasket is positioned between (a) said front end cap and saidpiston sleeve,

(b) said rear locking nut and said piston sleeve, and

(c) said rear locking nut and said shank member.

7. The piston assembly of claim 6 wherein said resilient,

refractory gasket is composed of expanded graphite.

References Cited UNITED STATES PATENTS 2,219,989 10/1940 Gimm 92-221 X3,277,797 10/ 1966 Tyree 92-252 X FOREIGN PATENTS 975,401 10/ 1961Germany.

1,108,231 l/l956 France.

MARTIN P. SCHWADRON, Primary Examiner.

G. N. BAUM, Assistant Examiner.

1. A PISTON ASSEMBLY FOR USE IN AN APPARATUS FOR DIE CASTING A HIGHMELTING POINT METAL COMPRISING: (A) A PISTON SLEEVE COMPOSED OF TITANIUMDIBORIDE, SAID PISTON SLEEVE HAVING AN ANNULAR OPENING THERETHROUGH; (B)A PISTON TIE ROD COMPOSED OF A METAL HAVING A LOWER COEFFICIENT OFTHERMAL EXPANSION THAN THE TITANIUM DIBORIDE PISTON SLEEVE EXTENDINGTHROUGH SAID ANNULAR OPENING IN SAID PISTON SLEEVE, SAID PISTON TIE RODHAVING A FRONT AND A REAR END;